1. Field of the Invention
The present invention relates to a method of producing a mirror for electromagnetic radiations of short wavelengths such as ultraviolet rays, soft X-rays, or X-rays, the mirror being used for an excimer laser generator and laser machining apparatus, X-ray microscope, X-ray telescope, or SR (synchrotron radiation) facility.
2. Description of the Prior Art
An X-ray microscope is provided with an X-ray mirror which needs to meet stringent specifications. X-ray mirrors in general use are manufactured by deposition of gold (Au) or platinum (Pt) on a substrate of artificial quartz or silicon carbide film formed by chemical vapor deposition. A mirror to reflect electromagnetic radiations of short wavelengths such as X-rays is based on the following two principles which are mentioned in SPIE vol. 315, p. 90-102, by D. H. Biderback, and Ouyo Butsuri (Applied Physics) vol. 56, No. 3, p. 342-351, by Aoki.
(i) Reflection of X-rays is by total reflection, which takes place when the oblique incident angle with respect to the mirror surface is smaller than the critical angle (.theta..sub.c) given by the equation (1) below. EQU .theta..sub.c =1.64.times.10.sup.-3 .sqroot..rho..multidot..lambda.(rad)(1)
.rho.: density of mirror substance (g/cc) PA0 .lambda.: wavelength of X-ray (.ANG.) PA0 R.sub.0 : Reflectivity of an ideal mirror having a surface roughness of 0 .ANG. PA0 .sigma.: surface roughness (.ANG.) PA0 .theta.: oblique incident angle of X-ray (rad) PA0 .lambda.: wavelength of X-ray (.ANG.)
It is apparent from the equation (1) that the higher the density (.rho.), the greater the critical angle (.theta..sub.c) for a given wavelength (.lambda.) of X-ray. For example, Au has a critical angle (.theta..sub.c) of approximately 3.2 for an incident X-ray having a wavelength of 8 .ANG.. Therefore, Au finds practical use.
(ii) Reflectivity (R) of X-rays depends on the surface roughness of the mirror as indicated by the equation (2) below. EQU R=R.sub.0 exp[-4.pi..sigma.sin.theta./.lambda.).sup.2 ] (2)
The equation (2) indicates that the surface roughness .sigma. should be as small as possible for a given wavelength (.lambda.) and incident angle .theta.). In the case of X-rays, the value of .sigma. should be smaller than 5 .ANG. for practical use.
It goes without saying that the foregoing argument can be applied also to soft X-rays and ultraviolet rays having longer wavelengths than X-rays.
The X-ray mirror based on the above-mentioned theory is produced by the conventional method explained in the following.
FIG. 6 shows a vacuum deposition apparatus cited from Kinzoku Hyomen Gijutsu (Metal Surface Technology), 1979, vol. 30, No. 5, p. 11. In the figure, there is shown a deposit material (Au) I, a glass substrate 2, an Au film 3 formed on the glass substrate 2, a crucible 4, a crucible heater 5, an evacuation system 6, a rotary shutter 9, a substrate holder 10 with a cooling mechanism, a vacuum chamber 11, and a power supply 12.
The apparatus shown in FIG. 6 is operated in the following manner to form the Au film 3 on the glass substrate 2. First, the glass substrate 2 and the crucible 4 containing the deposit material I are arranged in the vacuum chamber 11. The vacuum chamber 11 is evacuated by the evacuation system 6, so that the pressure in the vacuum chamber 11 is reduced to about 10.sup.-6 Torr. The crucible 4 is heated by means of the heater 5 connected to the power supply 12, so that the deposit material (Au) reaches about 1500.degree. C. at which Au has a vapor pressure of 0.1 to 1 Torr. With the deposit material heated, the shutter 9 is opened so that Au vapor condenses on the glass substrate 2 positioned above the crucible 4. During deposition, the substrate is usually kept lower than room temperature by passing water through the substrate holder 10 in order to prevent the Au film from becoming rough (presumably due to recrystallization) while it is being formed. When the Au film 3 has reached a desired thickness (usually 500-1000 .ANG., the shutter 9 is closed. The film thickness is usually measured with a thickness gauge of quartz oscillator type (not shown). This is the conventional method of producing a mirror.
Incidentally, it may be conceivable to use Au or Pt in bulk superfinished by a method of float polishing in place of an Au or Pt in film deposited on a substrate, if it is a sole object to obtain a high reflectivity.
The production of mirrors by the above-mentioned vacuum deposition apparatus has a disadvantage that adhesion of the Au film 3 to the glass substrate 2 is too low to make the mirror reliable for practical use. (The Au film is peeled off by a piece of adhesive tape.) This occurs if vacuum deposition is performed when the glass substrate 2 is not completely cleaned, because the deposit material 1 is given only a small amount of energy (approximately 0.1 eV) at the time of deposition. For improved adhesion, it is desirable to keep the glass substrate 2 at a high temperature; but this gives rise to an Au film having a rough surface which increases scattering. This is in contradiction to the above-mentioned requirement (i). On the other hand, vacuum deposition performed with the glass substrate 2 kept at a low temperature gives rise to an Au film having a low density. This is in contradiction to the above-mentioned requirement (ii). The glass substrate 2 held at a low temperature does not permit Au atoms striking the substrate to move to the stable lattice position, but permits Au atoms to form a film at the spot which they strike. The Au film thus formed has a low density and hence has a small critical angle for total reflection.
A mirror formed by polishing Au or Pt in bulk has the surface structure as shown in FIG. 7 which contributes to total reflection. The surface structure consists of crystal grains 13 and affected layers 14. In the case of a mirror of this type, it is difficult to reduce the surface roughness below 50 .ANG. even by the float polishing which produces the flattest surface available at the present time. This is because the working efficiency differs depending on the orientation of crystal grains. Moreover, it is impossible to completely eliminate the affected layer. As a result, the outermost surface layer of about 10 .ANG. thick (2- to 3-atom layer) contains residual abrasive grains used for float polishing and also has lattice strains remaining in the bulk surface. In addition, Au and Pt are too expensive to be used in bulk as a mirror.